Base saturation refers to the proportion of exchange sites occupied by which type of cations?

• A. Anions
• B. Acidic cations (H+ and Al3+)
• C. Basic cations (Ca2+, Mg2+, K+)
• D. Neutral cations (Na+, NH4+)

Answer: (C)

Base saturation is the proportion of the soil’s cation exchange capacity that is occupied by base cations—primarily calcium, magnesium, and potassium. The exchange complex on clay and organic matter holds charges; when most of those sites carry Ca2+, Mg2+, and K+, the soil is less acidic and usually more fertile. In acidic soils, hydrogen and aluminum dominate the exchange sites, lowering base saturation. Anions don’t fill these cation exchange sites, so they aren’t part of base saturation. In this context, the basic cations CA, MG, and K are the correct focus, which is why that option is the best choice. (Example: if the CEC is 20 cmol(+)/kg and base cations occupy 12 cmol(+)/kg, base saturation is 60%.)